In conventional current mode step-down DC-DC converters, on-duty ratio is determined by comparing a coil current waveform obtained by detecting a coil current and converting this current to voltage with an output signal from a differential amplifier. This on-duty ratio is a ratio of an ON period of a main switching element within an operating cycle of the DC-DC converter. In the case of such control, in an input/output relationship wherein the on-duty ratio is 50% or higher, slope compensation for preventing sub-harmonic oscillations is generally applied to the coil current waveform.
Related technology is disclosed in the Japanese Laid-open Patent Publication No. 2004-173353.
As the coil current waveform is modulated by slope compensation in case where an input voltage is changed in an input/output relationship wherein the on-duty ratio is 50% or higher, the value of the output signal from the differential amplifier changes. As the output signal from the differential amplifier changes, the DC-DC converter controls the output voltage based on the output signal from the differential amplifier. This destabilizes the output voltage.